Vehicles, for example, automobiles driven by an internal combustion engine (hereinafter also referred to simply as an “engine”) have a canister such as a charcoal canister as means of adsorbing and storing evaporative fuel gas generated in a fuel tank so as to prevent air pollution. The evaporative fuel gas adsorbed by the canister is purged to an intake passage of the engine at a flow rate controlled by a purge control valve (canister purge control valve). This kind of control is referred to as “canister purge control.”
Existing types of air-fuel ratio control devices for engines in which evaporative fuel gas purge control is performed as described above include one in which: the quantity of evaporative fuel gas purged from a canister is estimated based on the intake pipe negative pressure, engine speed, and purge control valve opening; the air-fuel ratio variation caused by purging of evaporative fuel gas is estimated; the quantity of fuel supply is corrected, while evaporative fuel gas is purged, based on the estimated value of air-fuel ratio variation; and feedback control is performed by applying a correction factor determined based on the air-fuel ratio detected by an air-fuel ratio sensor (see JP-A No. H10-141114, for example).